This invention relates to a device for properly setting a thread tension of a sewing machine by operation in response to conditions of a fabric to be sewn and a thread to be used.
With respect to various stitching conditions, an upper thread tension and a lower thread tension or a lower thread supplying amount are set to predetermined stitching conditions such that desired stitches may be produced by operation of fabric thickness selecting buttons (called briefly as "fabric sort button" hereinafter).
In sewing machines generally used, an operator adjusts an upper thread tension device so as to make proper crossing positions of the stitches of the upper and lower threads in accordance with data from the type of fabric, thread, feed amount, and thread tension being used. There have been many proposals for methods for automating the selection of the upper thread tension, however they have not taken into consideration the type of fabric and thread being used.
In some known sewing machines, complex devices for setting thread tension comprise an adjusting cam operating connected to pattern cams. A linkage which is connected to the adjusting cam and to a control cam is operated to adjust a lower thread tension in accordance with a respective pattern cam.